1. Field of the Invention
The present invention relates to a mass analysis method and an Inductively Coupled Plasma Mass Spectrometer (ICP-MS) generating atomic ions from samples through an ICP and performing mass analysis.
2. Description of Related Art
One of the devices that analyze elements containing in samples is an ICP-MS (e.g., Patent Document 1). The ICP-MS has the following strong points: for a wide range of elements for lithium to uranium (except for some elements such as rare gas), ng/L-degree ultramicro elements may be detected through ppt (parts per trillion) level analysis, for example, for quantifying many harmful metals (heavy metal elements) contained in environmental samples such as tap water or river water, and land, or quantifying many elements contained in food and drugs.
The ICP-MS has a plasma ionization part that generates atomic ions from samples (mainly liquid samples), through an ICP, and a mass analysis part that analyzes the generated atomic ions. The plasma ionization part has a plasma torch which has a sample gas tube for circulation of a sample gas, a plasma gas tube formed on the periphery of the sample gas tube, a cooling gas tube formed on the periphery of the plasma gas tube, and a high-frequency induction coil wound to a front end of the cooling gas tube. If a plasma gas such as argon flows in and a high-frequency current flows towards the high-frequency induction coil of the plasma torch, a plasma (6,000 K-10,000 K high-temperature plasma) is generated at the front end of the plasma torch. If a sample (e.g., an atomized liquid sample obtained through an atomizing gas) is introduced from the sample gas tube in the state, in the high-temperature plasma, compounds in the sample are atomized and ionized, to generate atomic ions. The generated atomic ions are guided to the mass analysis part and separated corresponding to mass-charge ratios.
In the ICP-MS, it is common to sequentially continuously analyze multiple (e.g., about 100) samples selected under the same or similar conditions with the same condition, and quantify about 20-30 target elements contained in each sample. Herein, the program of continuously analyzing multiple samples in the ICP-MS is described.
Firstly, one of the multiple samples is selected as a representative sample to be introduced into the plasma ionization part, and scanning measurement is carried out on the atomic ions generated from the representative sample. Accordingly, a mass spectrum of the representative sample can be obtained.
Secondly, the analyzer confirms the mass spectrum, infers elements contained in the representative sample according to the position (mass-charge ratio) of a mass peak in the mass spectrum, and extracts target elements (e.g., heavy metal elements) therefrom. (Natural) isotopes are present in many elements, and the presence ratio thereof is also known. Therefore, for an element, if a mass peak appears in the position of the mass-charge ratio corresponding to all isotopic ions, it may be inferred that the sample contains the element.
Then, the analyzer, for all the target elements, determines, from isotopic ions with different mass-charge ratios, through which isotopic ion (mass-charge ratio) the element is measured. At this point, when there are isotopes without other ions (hereinafter referred to as “interference ions”) with the same mass-charge ratio (i.e., a mass peaks of other ions do not overlap), the isotopes are used for measurement. The other ions (interference ions) herein include: other element ions (isobar ions), compound ions (oxide ions, chloride ions, plasma gas adduct ions, etc.), and multivalent ions. On the other hand, when interference ions are present in all isotopes of the target elements, isotopes with a small number of interference ions or small intensity of overlapping a mass peaks are used for measurement.
If isotopes for measurement are determined for all the target elements respectively according to the mass spectrum of the representative samples, target elements contained in each sample are measured through Selected Ion Monitoring (SIM) measurement using a mass-charge ratio (referred to as “measurement mass-charge ratio”), and the element is quantified according to intensity of a mass peaks of the target elements. SIM measurement using the measurement mass-charge ratio determined according to the mass spectrum of the representative sample is performed on multiple samples, and the target elements contained in the samples are quantified.